This invention relates to fuel cell batteries of the filter press-type design in general, and more particularly to an improved construction for such battery. Fuel cells using a liquid electrolyte and at least one gaseous reactant with the electrolyte and gas chambers of each of the individual fuel cells separated from each other by asbestos diaphragms and the individual fuel cells separated from each other by separator sheets, have been developed. Examples of such fuel cells are disclosed in German Offenlegungsschrift No. 2,026,220. As disclosed therein, the asbestos diaphragms have an electrolyte-impermeable and gastight outer zone of increased thickness. The outer zone of increased thickness forms a peripheral area which defines chambers for the gas or electrolyte when the asbestos diaphragms are pressed together directly or against the separator sheets separating the individual cells. Supply ducts for the electrolyte and the gas or gases to circulate them through the chambers are formed in the reinforced outer zones. As more fully disclosed in the above referenced Offenlegungsschrift, this design avoids the disadvantages associated with the plastic frames previously used in fuel cell batteries of this type and in particular, makes unnecessary the use of additional sealing elements such as sealing rings and the previously required plastic frames, thereby permitting a fuel cell which is thinner and of a much simpler design.
Within the electrolyte and gas chambers defined by the reinforced peripheral outer zones of the asbestos diaphragms, the asbestos diaphragms themselves and the separator sheets, electrodes and support frames are inserted when the battery is assembled.
One type of a fuel cell battery previously disclosed has fuel cells with a liquid electrolyte for reacting gaseous reactants such as hydrogen and oxygen. In this type of fuel cell, an electrode in powder form is placed in each of the gas chambers. That is, in this construction, two gas chambers are provided, one on each side of an electrolyte chamber. The electrolyte chamber has a support frame situated therein which maintains proper spacing of the two asbestos diaphragms forming the sides of the chamber. These two asbestos diaphragms, respectively, form one side of each of the gas chambers with the other side of the gas chamber defined by a separator plate. The electrode in powder form in each of the gas chambers is pressed toward the support frame by metal screens with the pressure transmitted through the asbestos diaphragms.
Another embodiment of a fuel cell battery has fuel cells in which a fuel such as hydrazine is dissolved in an electrolyte and is reacted with a gaseous oxidant such as oxygen. In fuel cells of this nature, a fuel electrode (anode) for example, in the form of a screen electrode, and a support frame are arranged in the electrolyte chamber. In this arrangement, each cell includes only one gas chamber associated with the electrolyte chamber. The gas chamber is separated from the electrolyte chamber by an asbestos diaphragm with an oxidant electrode (cathode) such as an oxygen gas diffusion electrode placed in the gas chamber on the other side of the asbestos diaphragm. Thus, the asbestos diaphram forms one side of the electrolyte chamber and one side of the gas chamber. The other sides of the respective chambers are formed by separator sheets which are in electrically conducting contact with adjacent electrodes.
It has been discovered that in the manufacture of the individual parts of batteries of this nature, i.e., the manufacture of parts such as asbestos diaphragms, electrodes and support frames, only certain manufacturing tolerances can be obtained in a practical manner. This is particularly true when constructing the fuel cell batteries of large output ratings in the range of several kilowatts and which require several hundred fuel cells in a single battery. Because of the large number of individual parts required in such a battery, for economic reasons, it is necessary that tolerances not be too close. Otherwise, the cost of manufacturing would be too great.
In fuel cell batteries of this nature, the pressure in the gas chambers is generally higher than the pressure in the electrolyte chambers. For example in a hydrazine/oxygen battery, the pressure in the electrolyte chamber will be about 15 N/cm.sup.2 and in the gas chamber, which is separated from the electrolyte chamber by an electrolyte-saturated, gastight asbestos diaphragm, about 20 N/cm.sup.2. The pressure difference between the pressure in the gas chamber and in the electrolyte chamber is counter-acted by the contact pressure of the approximately equal electrodes and/or support frames. Thus, the asbestos diaphragm will be stressed only in compression, and there will be no shear forces on the area of the asbestos diaphragm between two adjacent components. This can be seen by reference to FIG. 10 in which an asbestos diaphragm 100 separting a hydrazine electrode 104 in an electrolyte chamber and an oxygen electrode 105 in a gas chamber. Over the distance designated L, the asbestos diaphragm 100 is supported on both sides by the respective adjacent components. In the prior art, the support frames and electrodes were made of approximately the same size. Thus, in the prior art, the hydrazine electrode 104 would have been terminated at the line 120. In such a case, the enlarged reinforced outer zone of the diaphragm would have been equal on both sides as shown by the dotted lines. Because of the above noted manufacturing tolerances, a gap G remains between the electrode and the edge such as edge 108 of the reinforced outer portion of the diaphragm. Gaps in the range of 0.1 to 3 mm will be found in typical fuel cells. In this gap area G, there is a pressure towards the electrolyte chamber in the direction of arrow 121 due to the higher gas pressure. As a result, shear forces develop which are a function of the gap width and the prevailing pressure difference. These forces acting for an extended period of time can lead to damage of the asbestos diaphragm, particularly to tearing of the diaphragm in the vicinity of the gaps. Thus, it can be seen that there is a need to eliminate these problems by providing an improved design in which such shear forces are not present.